The present invention relates to a method and apparatus for the transportation of hazardous wastes in which the hazardous waste includes both solid and liquid components mixed together. The invention provides for thorough mixing of the solid and liquid components with numerous pertinent advantages.
As people and companies become more and more attuned to the appropriate disposition of hazardous waste, whether by personal desire or government regulation, the need arises for finding suitable ways of handling these wastes, so that the disposition can be effectuated safely and economically.
A particular class of hazardous waste has a high volatility and combustibility, making it dangerous to handle but useful as a fuel. An example of this type of waste is generated by the painting process. Leftover paints, paint thinners, paint solvents, and paint cleaning compositions are mixed together to yield a mixture which has these characteristics and which is toxic when released to the environment. An industry has arisen to use these paint-related wastes as fuel in industrial burners, most particularly in cement kilns. The kilns burn the solid and liquid waste as part of the burner fuel, and the ash residue of the combustion process serves as clinker in the cement. The high temperature in the kiln safely decomposes the various components of the paint-related residue to benign discharges. In addition, many of the solvent components of the waste are recyclable by removal from the waste by distillation. This reduces the volume of the waste, but increases the proportion of solids.
However, the kilns involved are located at some distance from the origin of the waste or the place of distillation, so a need arises to transport the waste to the kiln. Since the waste is highly combustible, transportation of the waste, by United States Department of Transportation regulations, must be in tank trucks having special construction. The applicable standard is Department of Transportation Hazardous Waste MC 307, which includes requiring that the empty tank not leak when subjected to an air pressure of 1.76 kilograms per square meter, so that the structural integrity of the tank must be substantial. Any breach of the wall of the tank requires retesting of the tank to its MC 307 rating. The entire text of the Department of Transportation MC 307 standard is incorporated herein by reference.
At the completion of the trip to the kiln, the discharge of the hazardous waste from the tank to the kiln burner or a holding tank at the kiln has previously been problematic. The liquid successfully drains from the tank, but a large quantity of the solids remain. Previous attempts to deal with this problem include providing drop centers or drop bellies in the tanks so that the bottom wall of the tank is sloping, and gravity assists in the discharge of solids along with the liquids. This design has met with only limited success.
The retention of solids in the bottom of the tank poses numerous disadvantages. First, the backhaul of the tank to pick up another load entails hauling the solids back to the pickup point, making the tank heavier and wasting fuel. Of course, the buildup of the solids reduces the capacity of the tank so that each subsequent refill of the tank includes less and less volume. Typically, two inches of solids were deposited in the bottom of the tank from each load prior to the use of the present invention.
Also, the solids have a high BTU level, so that they contribute to the fuel value of the hazardous waste when they are adequately discharged with the liquid. For them to remain deposited in the bottom of the tank diminishes the available fuel value of the waste.
Moreover, the proper operation of the kiln requires the proper mix of components in the hazardous waste when used a fuel, including maximum allowable levels of chloride and water, and other components. Not only are the solids capable of precipitating from the mixture, but some of the liquids in the mixture are not entirely miscible. These liquids can separate, causing the liquid as discharged from the tank to have different fuel values, depending on which portion of the tank is being drained at any given time. This can be disruptive to proper operation of the kiln or proper complete combustion of the waste so that the kiln's discharges fall outside of permissible or desired ranges. As a result, the kiln operator tests each load of waste and sometimes turns away a load found not to meet its specifications.
Some such tests have been erroneous in that only a certain fraction of the overall volume of hazardous waste was sampled because of the separation of components. Although the overall mixture in the tank may be within the parameters set by the kiln, the precise sample taken by the kiln tester may fall outside these parameters and result in rejection of the entire truckload.
Typically, the buildup of the solids in the bottom of the tank has been dealt with by having a workman enter the tank with a shovel to shovel the solids into barrels for processing. Such processing, rather than being a useful disposition of the solids as in burning in the kiln, constitutes underground storage or wasteful incineration. This does not, of course, solve the problems of incomplete mixing of liquids and is not a chore the workman typically enjoys. It is known that a previous attempt to agitate the contents of a hazardous waste transport tank included the use of vertically extending augers or the like, but it resulted in a considerable residue of solids in the tank after it was supposedly drained. While other types of tanks in which agitators are provided are known, such as concrete mixers and the like, none are designed for the hauling of hazardous waste under conditions meeting Department of Transportation MC 307 standards.
Thus, the settling of the contents of the tanks has posed numerous problems, which prior art attempts to rectify have not been successful in alleviating. Accordingly, there is a need in the art for a method and apparatus for agitating the hazardous waste solids and liquids to maintain them in flowable form so that they may be fully and usefully discharged from the tank.